In 2002, I checked the error concealment ability of my drives and CD player. Here is the result : http://perso.numericable.fr/~laguill2/dae/interpolation/interpolation.htm
I saw the CD-ROM drives tested use this method :
Hold the value of the last good sample until the sample prior the last bad one.
Give the last bad sample the average value between the held value and the first good sample after the error.
The CD player (from 1991) applied a similar method, exept that the holding was done until the 9th sample before the next good one. Then, the 8 last samples of the error undergo a linear interpolation between the held value and the next good sample.
In order to get the best from some CDS200 CDs, you need a drive with good error correction ability. In my test (homepage : http://perso.numericable.fr/~laguill2/dae/dae.htm ), it would be the Teac e540, because it corrects up to 4 C2 errors per frame, and it does not flag the whole C1 frame as uncorrectable when it can’t get it right (that what I called “use of EFM”, though I don’t know if this is right). But this drive have another problem, it sometimes slightly offsets several seconds of audio, and, the transition occuring in the middle of the wav file, this is very audible.
However, I’ve noticed recently some CDS200 CDs that are plagued with many errors not being just mastered C2 errors. No way to get the same data twice from them. They produce annoying audible clicks even in my hi-fi player.
Looking closely at what’s happening, I discovered that these clicks are not CU errors. They are missing blocks !
Looking at the waveform, it is interleaved with itself for a short time, the data alterning samples from time t with samples from time t+6, then is goes on properly, but offsetted 6 samples backwards ! One full block of data was omitted.
Reading a second time, the missig block was there, but other blocks were missing at other places. There is something in this system that makes drives miss blocks. They probably have tampered with the synchro headers of the blocks.
The easiest way to deal with this problem is to ask EMI for a correct version of the CD.